1. Field of the Invention
The present invention relates to a control for an internal combustion engine and, more particularly, to an air-fuel ratio control for an engine.
2. Description of Prior Art
In internal combustion engines, particularly in engines of electronically controlled fuel injection type, engine operating conditions, such as engine loads and engine speeds, are classified into a plurality of regions wherein fuel supply is controlled such that different air-fuel ratios are established among different regions. For example, in Japanese patent application No. 56-170949 filed on Oct. 26, 1981 and disclosed for public inspection of Apr. 30, 1981 under the disclosure no. 58-72631, there is disclosed an engine air-fuel ratio control system wherein the engine operating range is divided into a high speed, heavy load region and a low speed, light load region and the fuel supply is controlled in the high speed, heavy load region under the feedback signal from a O.sub.2 sensor provided in the exhaust system so that a stoichiometric air-fuel ratio can be established. The low speed, light load region is further divided into a plurality of sub-regions and the fuel supply is controlled in the sub-regions so that an air-fuel mixture leaner than the stoichiometric ratio is established. According to the teachings in the Japanese patent application, the fuel supply control is such that the leanest mixture is produced in the sub-region which is furthermost from the high speed, heavy load region and the mixture is made gradually closer to the stoichiometric value in the regions closer to the high speed, heavy load region.
The control system disclosed in the Japanese patent application is considered as desirable in that an abrupt change in the air-fuel ratio can be avoided between the high speed, heavy load region and the low speed, light load region. It should however be noted that the system is not satisfactory because a sufficiently lean mixture cannot be produced in the sub-region adjacent to the high speed, heavy load region so that a sufficient improvement cannot be achieved in fuel economy. In Japanese patent application No. 55-181210 filed on Dec. 23, 1980 and disclosed for public inspection on July 1, 1982 under the disclosure no. 57-105530, there is disclosed a control system wherein the air-fuel ratio is changed at a constant rate when the engine operating condition is changed from the high speed, heavy load region to the low speed, light load region, and vice versa. In this control system, it will not be necessary to divide the low speed, light load region into a plurality of sub-regions so that it may be possible to improve the fuel ecomomy. It should however be noted that, since the air-fuel ratio is changed at a constant rate, the air-fuel ratio cannot always be controlled as desired. For example, when the engine throttle valve is opened for acceleration in the low speed, light load region, the air-fuel ratio may not be enriched rapidly so that the operator may feel lack of engine power.